1. Field of the Invention
The present invention relates to an optical device for use in receiving and detecting a returned light reflected from an irradiated portion by irradiating a light from a light-emitting portion, for example, on the irradiated portion of an optical recording medium such as an optical disk, a phase-change type optical disk and so on, and particularly to an optical device for use in detecting a tracking error signal relative to an optical disk having a pit depthxcex/4n or a recording portion equivalent thereto.
2. Description of the Related Art
In optical devices such as an optical pickup of an optical disk drive of a so-called compact disc (CD) player and a magnetooptical disk drive, respective optical assemblies such as a grating, a beam splitter and so on are individually fabricated so that an overall arrangement of a device becomes complicated and large. Moreover, when optical assemblies are fabricated on a base in a hybrid fashion, optical assemblies should be disposed with a strict alignment accuracy.
FIG. 1 is a structural diagram showing an example of a conventional optical pickup 81 that is exclusively used for reproducing a compact disc (CD). This optical pickup 81 comprises a semiconductor laser 82, a diffraction grating 83, a beam splitter plate 84, an objective lens 85 and a light-receiving element 86 composed of a photo-diode. A laser light L from the semiconductor laser 82 is reflected on the beam splitter plate 84, converged by the objective lens 85 and thereby irradiated on an optical disk 90. A returned light reflected on the optical disk 90 is traveled through the objective lens 85 and the beam splitter plate 84 and received and detected by the light-receiving element 86.
However, such optical pickup 81 has not only many assemblies and become very large in size but also many assemblies thereof should be disposed with a high accuracy so that its productivity is low accordingly.
As a tracking servo method in an optical device such as an optical pickup or the like, there are generally used a push-pull method, a 3-beam method, a heterodyne method and the like.
Of these methods, according to the conventional push-pull method, when a beam spot of incident light on a disc is displaced from a track or a pit, a difference of intensity occurs in +first-order light and xe2x88x92first-order light, whereby a far field pattern (FFP) becomes asymmetric. Then, two photo-detectors, for example, detect signals corresponding to the asymmetric far field pattern, and a computing device computes these signals to detect a displacement of a beam spot (see FIG. 2).
FIGS. 2A and 2B are each a schematic structural diagram showing a tracking servo using a push-pull method.
As shown in FIG. 2B, when a light is irradiated on concavities and convexities of pits formed on the surface of a disk 52, the concavities and the convexities diffract the light to provide a 0-order diffracted light (main beam B) and xc2x1first-order diffracted lights (sub-beams Bxe2x80x2).
In FIG. 2B, reference numerals S0, S1 denote irradiated spots of the 0-order diffracted light and the xc2x1first-order diffracted lights, respectively. The irradiated spot S0 becomes circular because of an aperture of an objective lens.
In this case, as shown in FIG. 2A, two split photo-diodes PDR, PDL are disposed as a light-receiving unit. Lights received by these photo-diodes PDR, PDL are computed by a suitable device such as a differential amplifier or the like, not shown, like (PDLxe2x88x92PDR), for example, to thereby obtain a tracking error signal TE as a tracking signal.
When the track and the center axis of incident beam are shifted from each other, there is caused a difference in diffracted information between xc2x1first-order diffracted lights, so that TE=(PDLxe2x88x92PDR) does not become zero but indicates a positive or negative value in response to the shifted direction. Thus, it is possible to detect the direction and the amount in which the center axis of the incident beam is shifted from the track.
Although the tracking servo system using the push-pull method may be realized by the two split photo-diodes and may be made inexpensive, there arises a problem that, when the lens is shifted, the returned light from the disk is vertically shifted on the light-receiving surface with respect to the split line of the light-receiving element, thereby resulting in a large offset being generated in the signal.
As shown in FIG. 3A, when a lens 51 is shifted in the lateral direction, spots of lights received by the photo-diodes PDL, PDR are shifted as shown by dashed line concurrently therewith. Thus, even when the tracking is properly made, the tracking error signal TE=0 is not satisfied.
Also, as shown in FIG. 3B, when the lens 51 is skewed relative to the disk 52, spots of received lights are also shifted as shown by dashed lines. Thus, even when the tracking is properly made, the tracking error signal TE=0 is not satisfied.
FIG. 4 shows measured results of influences exerted upon the tracking error signal by the lens shift in the case of the conventional push-pull signal as described above. Incidentally, the vertical axis represents measured results in the form of a relative value. Influences were computed by using a disk such that a groove pitch was 1.60 xcexcm, a groove depth (depth) was wavelength/8 and a duty (duty:groove ratio) was 65%. Also, a wavelength was 0.78 xcexcm.
A study of FIG. 4 revealed that, according to the conventional push-pull signal, when the lens is shifted, the whole of the tracking error signal also is shifted concurrently therewith.
According to the push-pull method, when a wavelength of reproducing laser is xcex and a refractive index of the transparent base of the disk 52 is n, if a depth of pits on the disk 52 is xcex/4n, then a signal due to interference between the 0-order diffracted light and xc2x1first-order diffracted lights becomes zero. As a consequence, it becomes impossible to detect the tracking error signal from a principle standpoint. Accordingly, the push-pull method may not be applied to the standardized disk 52 in which the pit depth is xcex/4n.
For example, a DVD (Dedital Versatile Disk)-ROM and a DVD-Video have the pit depth of xcex/4n according to the standards, so that the push-pull method may not be applied to such disks.
According to the three-beam method, the diffraction grating separates a light to provide a main beam and two sub-beams on both sides of the main beam. FIG. 5 shows positions of spots formed on the disk surface according to the three-beam method. As illustrated, reflected beams of two sub-beams are respectively detected by irradiating a spot S0 based on the main beam and spots S1, S2 based on the sub-beams on both sides of the main beam on the grooves or pits of the disk 52 and a difference signal is calculated, thereby effecting a tracking servo similar to that of the push-pull method.
When the spot S0 of the main beam is shifted from the center of the track, the reflected lights based on the spots S1, S2 of the sub-beams become asymmetrical so that the tracking error signal provided by the difference signal is fluctuated from zero. Since the fluctuated amount of this tracking error signal changes in accordance with the amount in which the spot S0 of the main beam is shifted from the center of the track, there may be effected the tracking servo.
Incidentally, the reflected light of the main beam is used to detect a disk recording signal.
Although the three-beam method may cope with the above-mentioned lens shift, the three-beam method has the drawback such that the light should travel through the diffraction grating such as a grating or the like, causing the number of assemblies to increase, an amount of light of the main beam decreases, causing a power consumption to increase, an adjustment is complicated, requiring much manufacturing cost and so on.
Further, as a method which is effective for the tracking servo of the standardized disk in which the pit depth is xcex/4n, there is known a phase difference detection method.
The phase difference detection method is realized by a method of detecting a diffraction spectrum of a two-dimensional pit by heterodyning with reference to an RF (higher harmonic wave) signal or a method of digitally computing each signal detected on the photo-detector.
According to the phase difference detection method, as shown in FIG. 6, for example, quadrant photo-diodes PD1, PD2, PD3, PD4 are formed about the optical axis with respect to the tangential direction T of the pit series direction, for example, of the optical disk serving as the irradiated portion and the direction perpendicular to this direction T, and located in the far field region. Then, a returned light from the optical disk is detected by the quadrant photo-diodes PD1 to PD4.
In FIG. 6, a center circle corresponds to the pupil of the lens, and is equivalent to the spot of the 0-order diffracted light. Other eight circles surrounding the center circle are equivalent to the spots of the first-order diffracted light. Also, a central dashed-line block is an image corresponding to the pit P on the disk.
Then, with respect to the arrangement of the quadrant photo-diodes PD1 to PD4, the following signal processing will be executed, for example.
An RF signal (PD1+PD2+PD3+PD4), which is the sum total of the detection signals of the respective photo-diodes and a signal (e.g. PD1+PD3xe2x88x92PD2xe2x88x92PD4) which results from computing the detection signals of the respective photo-diodes are detected by heterodyning, taking a phase into a consideration.
A content of a computed signal obtained at that time is shown by the Expression (1):                                                                         Computed                ⁢                                  xe2x80x83                                ⁢                signal                            =                                                (                                                            PD                      1                                        +                                          PD                      3                                                        )                                -                                  (                                                            PD                      2                                        +                                          PD                      4                                                        )                                                                                                        =                              C                ⁢                                  xe2x80x83                                ⁢                                  sin                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                                                                        v                          t                                                /                                                  v                          p                                                                                      )                                                  ⁢                                  sin                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                      a                      ⁢                                              xe2x80x83                                            ⁢                      ω                      ⁢                                              xe2x80x83                                            ⁢                                              t                        /                                                  v                          p                                                                                      )                                                                                                          (        1        )            
where
Vt: detrack amount
vp: cycle of pit series
a: radius of read-out position
xcfx89: angular velocity of optical disk
C: constant
Having considered that RF signal (PD1+PD2+PD3+PD4) is cos(2xcfx80axcfx89t/vp), C sin(2xcfx80vt/vp) on the equation (1) becomes a signal which results from detecting the computing signal by heterodyning with reference to this phase. The detrack amount vt may be computed from the signal which was obtained by heterodyning in this way.
In this case, the tracking signal is made difficult to offset by the lens shift. Also, this method is effective for the standardized disk in which the pit depth is xcex/4n.
On the other hand, in order to improve the defects encountered with the above-mentioned conventional optical device, there has hitherto been a so-called CLC (confocal laser coupler) in which optical assemblies may be reduced and an alignment with which optical assemblies are located may be simplified and in which a light-emitting unit is located at a confocal position of a converging means such as a lens or the like and a light-receiving unit is formed near the confocal position at which this light-emitting unit is located in order to simplify and miniaturize the overall arrangement of the device.
In order to remove the offset caused in the tracking error signal by the aforementioned lens shift and the disk skew, the assignee of the present application has previously proposed an optical device in which split photo-diodes forming a light-receiving unit are disposed at the above-mentioned confocal position and in which these split photo-diodes execute the tracking servo by using a push-pull method or the like (see xe2x80x9cOPTICAL DEVICExe2x80x9d described in Japanese patent application No. 7-35528).
According to such an optical device, since the tracking error signal is detected by the push-pull method (CPP method) based on the light-receiving unit located near the confocal position, the tracking error signal may be detected stably against the lens offset and the disk skew and the alignment required in the assembly may be simplified considerably. Moreover, since the light-emitting unit and the light-receiving unit are formed on the same base, the number of assemblies may be reduced, a manufacturing cost may be reduced, and the optical device may be made highly reliable.
However, the above-mentioned CPP method has the defect inherent in the confocal optical system.
In particular, this defect directly becomes remarkable when an image is not properly focused and is very slightly defocused in the amount approximately within a focal depth.
FIG. 7 shows its example. FIG. 7 shows an example of numerically-computed results and explains a relationship between a tracking error signal and a detrack amount obtained by the CPP method when an image is defocused. An optical disk used is of the disk of the same shape that was used in the computation of FIG. 4.
A study of FIG. 7 reveals that, even when a defocus (generally, a term xe2x80x9cdefocusxe2x80x9d is not used within the focal depth but used for convenience"" sake) of less than xc2x11 xcexcm, e.g. approximately the same amount obtained within the focal depth or defocus within the focal depth occurs, the tracking error detection based on the CPP method causes an error.
Moreover, as in the case in which defocus=xe2x88x920.50 xcexcm is obtained as shown by a curve G in FIG. 7, there are generated a signal having a frequency different from that of an original tracking error signal (defocus=0.00 xcexcm shown by a curve E) or other tracking error signals, e.g. tracking error signal having a double frequency. Also, it should be noted that polarities of the signals are inverted as shown by curves H and I.
On the other hand, the optical system for the optical disk requires the focusing control as well as the tracking control when a signal is recorded/reproduced. In general, when a focusing is controlled, a defocusing amount is suppressed to become approximately less than a focal depth of an objective lens by a suitable method such as a spot size method, an astigmatism method, a knife edge method or the like. However, the defocus amount is not constantly eliminated to 0 xcexcm and is incessantly fine fluctuated within the focal depth. Accordingly, when a tracking error is detected by the CPP method, before discussing the pit depth, there should be adopted a correction method or a detection method which considers the influence of defocusing.
In view of the aforesaid aspect, according to the present invention, there is provided an optical device such as an optical pickup in which the number of optical assemblies may be reduced or the like, an alignment required when optical assemblies are disposed may be simplified, the overall arrangement of the device may be simplified and miniaturized, a tracking signal such as a tracking error signal or the like may be obtained stably for optical recording media having various pit depths and which may be manufactured with ease by a semiconductor process.
An optical device according to the present invention includes an irradiated portion composed of an optical recording medium in which pits are formed on a reflection surface, a semiconductor unit in which a semiconductor laser, a semiconductor structure comprised of a plurality of reflection surfaces and a photo-detecting element are formed on the same semiconductor substrate, and a converging a means for converging a light emitted from the semiconductor laser and irradiating the same on the irradiated portion and which further converges a returned light reflected from the irradiated portion. The semiconductor structure comprises at least a first reflection surface for reflecting the light from the semiconductor laser, and second and third reflection surfaces for irradiating a part of returned light from the converging means on the photo-detecting element. The first, second and third reflection surfaces of the semiconductor structure are formed near the confocal of the converging means. Also, the photo-detecting element comprises a first detecting element for receiving a returned light reflected on the second reflection surface and a second detecting element for receiving a returned light reflected on the third reflection surface in which a first detection signal is obtained by detecting a diffracted light from one pit edge of the pit formed on the optical recording medium by the first detecting element and a second detection signal is obtained by detecting the diffracted light by the second detecting element, and a tracking error signal is obtained by computing the first and second detection signals.
According to the above-mentioned arrangement of the present invention, since the first detection signal is obtained from the diffracted light from one pit edge of the pit formed on the optical recording medium by the first detecting element and the second detection signal is obtained from the diffracted light by the second detecting element and the tracking error signal is obtained by computing the first and second detection signals, it is possible to detect a tracking error signal which may satisfactorily cope with the standardized disk in which the pit depth is xcex/4n. Also, there may be obtained a tracking error signal which is difficult to be affected by the lens shift and the defocusing.
Further, since the semiconductor laser, the reflection surfaces and the photo-detecting element are formed on the same semiconductor substrate, the optical device may be formed by few assemblies, simplified and miniaturized.
An optical device according to the present invention comprises an irradiated portion formed of an optical recording medium in which pits are formed on a reflection surface, a semiconductor unit in which a semiconductor laser, a semiconductor structure composed of a plurality of reflection surfaces and a photo-detector element are formed on the same semiconductor substrate, and a converging means for converging a light emitted from the semiconductor laser, irradiating the same on the irradiated portion and further converging a returned light reflected from the irradiated portion, wherein the semiconductor structure comprises at least a first reflection surface for reflecting a light emitted from the semiconductor laser and second and third reflection surfaces for irradiating a part of a returned light from the converging means on the photo-detector element, the first, second and third reflection surfaces of the semiconductor structure are formed near a confocal of the converging means, the photo-detector element comprises a first detection element for receiving the returned light reflected on the second reflection surface and a second detection element for receiving the returned light reflected on the third reflection surface, the first detection element detects a first detection signal from a diffracted light obtained from one pit edge of the pits formed on the optical recording medium, the second detection element detects a second detection signal from the diffracted light, and the first detection signal and the second detection signal are computed to generate a tracking error signal.
According to the present invention, in the above-described optical device, the first reflection surface, the second reflection surface and the third reflection surface of the semiconductor structure are each formed of a predetermined crystal plane grown on the semiconductor substrate as a crystal.
According to the present invention, in the above-described optical device, the semiconductor structure is shaped as a triangular pyramid comprising the first reflection surface, the second reflection surface and the third reflection surface.
According to the present invention, in the above-described optical device, the first photo-detection element and the second photo-detection element are quadrant photo-detection elements.
According to the present invention, in the optical device, the semiconductor substrate has a concave portion of a predetermined depth retreated from the substrate surface of the semiconductor substrate at its portion in which the semiconductor laser is formed and the photo-detection element is formed in the concave portion.